1. Field of the Invention
The present invention relates to a method and apparatus insertable into a tubular pressure containing pipe (such as in an oil well), caisson, silo riser or conductor for separating liquid from an upward flowing liquid/gas multi-phase stream. More particularly, the method and apparatus are capable of providing a solution to the problem of eliminating and removing liquids from a multi-phase well or riser system where the build up of liquids can cause a significant loss of production.
FIG. 1 is a example schematic illustration of a typical hydrocarbon well completion. The well is not shown to scale. A multi-phase producing well such as illustrated in FIG. 1 may have its wellhead located on the sea-bed or on a platform or on land. For simplicity the invention described has the wellhead shown on the surface.
A well 1 has a production casing 2 at the top of which is secured a wellhead 3 and a tree 4. A production tubing string 5 is suspended within the casing 2. A tubing tail pipe 7 extends through a packer 8 into the live well above the shoe 6 on the casing 2 from the bottom of the production tubing 5. A smaller diameter casing liner with a shoe 9 may be positioned below the first shoe 6. Multi-phase hydrocarbon/water mixture from gas-bearing strata or zones 10, often many thousands of feet below the surface 11, enters the well above the shoes 6, 9 through appropriate ports or perforations indicated at 12 and flows upwardly through the tail pipe 7 and the tubing 5, via a sub-surface safety valve 13, into the tree 4 and from there through appropriate piping 14, to an export facility (not shown). The multi-phase flow enters above the shoes 6, 9 as indicated by the arrows, together with gas, liquid and vapour, at say a formation pressure PF. Additional condensation of liquid can form above the shoes 6, 9 and in the tubing 5 and results in a significant increase in density resulting in pressure PW at the bottom of the tail pipe 7, ultimately, resulting in a hydrostatic back pressure PH which reduces the production efficiency and may rise to a value which equals the formation pressure PF. At this point production ceases making the well non-productive.
2. Description of Related Art
Attempts have therefore been made to avoid the problem and, conventionally, as shown schematically in FIG. 2, this has been achieved by means of a downhole cyclone 15 being provided at the bottom of the production casing 2, together with an electric motor/pump combination 16/17. Dual tubing strings 18, 19 are provided, with liquid being pumped, for example, to the surface, through the tubing string 18 and gas, separated in the cyclone 15, passing through the tubing string 19. A system of this general type is described in U.S. Pat. No. 6,033,567. However, one of the problems with this conventional solution is that of pump replacement. Continuously running pumps used for the purpose currently have an average run life of about 12 months so that, on a fairly regular basis, the electric motor and pump 16, 17 have to be replaced, requiring what is known as a workover to be carried out. This involves removal of the tubing strings and is an expensive and time-consuming operation which shuts down production for a significant matter of time. For low liquid volumes, the pump would have to be stopped and started repeatedly. A further problem arises in controlling the pump 17. A sensitive measuring system is required to switch off the pump to prevent gas being drawn in in the event of liquid removal being temporarily completed.
There is a need therefore for a less complex and more efficient system of liquid separation.